"We've seen once-insuperable barriers fall in presidential politics -- and probably none more significant than a divorced man (Reagan) garnering the evangelical vote in 1980," said Randall Balmer, professor of religious history at Barnard College and author of God in the White House: How Faith Shaped the Presidency from John F. Kennedy to George W. Bush.

"Reagan, however, exuded an air of piety -- how sincere or not I still can't figure out -- whereas Trump seems to have a severe piety deficit," Balmer said. "This seems especially debilitating for someone with his sights set on the Republican nomination, where some semblance of faith seems to be a prerequisite. On the other hand, if (Newt) Gingrich can pull it off, I suppose anyone can."